Palladium Nanoparticles Electrodeposition onto Glassy Carbon from a Deep Eutectic Solvent at 298 K and Their Catalytic Performance toward Formic Acid Oxidation

A three-electrode electrochemical cell was used (100 mL capacity) fitted with a Lauda RMS Circulator with RM6 Refrigerating Water Bath Chiller, to set the cell temperature at 298 ± 0.24 K. The working electrode was a glassy carbon electrode (GCE), with 0.1963 cm² exposed area; the counter electrode was a platinum wire and a silver wire was used as pseudo reference electrode (Ag QRE), to which all potentials were quoted. The palladium nucleation and growth mechanism onto a GCE from the DES, was explored through electrochemical tests such as cyclic voltammetry (CV) and chronoamperometry (CA). The GCE was polished with diamond spray down to 0.25 μm, sonicated in methanol for 30 minutes, and finally rinsed with acetone to remove any residual particles and improve the surface adherence of the electrodeposited metal. CV and CA tests were carried out in an AutoLab PGSTAT100 potentiostat-galvanostat coupled to a PC running the NOVA 2.1.3 software for data acquisition and experimental control. These tests were carried out for both pure DES and Pd(II)/DES solutions.

Morphology of palladium deposited on the GCE from DES was obtained by means of Scanning Electron Microscopy (SEM) using a model Nova-2000 Nanolab double beam FEI SEM instrument with secondary electrons detector.

The deposit composition was determined by X-ray photoelectron spectroscopy (XPS) measurements, performed in an Escalab 250 Thermo Scientific (base pressure ∼2 × 10⁻⁹ mbar) equipment with an Al filament emitting X rays at 1486.6 eV. The Avantage 5 software was used for fitting the XPS profiles with Gaussian-Lorentzian (GL) line shape functions, calibrated by assigning a binding energy (BE) value of 285 eV to the C1s.

Figure 1 shows a typical CV recorded on the GCE working electrode in choline chloride-ethylene glycol eutectic mixture containing 5 mM PdCl₂ at 298 K. A well resolved peak clearly formed during the cathodic scan, associated with the reaction Pd(II)_DES + 2e⁻ = Pd_(s), displaying a current density, j_cp, that increased linearly with the square root of the potential scan rate, v, as shown in the inset in Figure 1, as described by the Berzins-Delahay equation for the reversible and diffusion-controlled deposition of metals onto solid electrodes,⁴⁰ which for 298 K corresponds to Eq. 1.

Where z is the number of electrons trasferred during Pd nucleation, C₀ (mol L⁻¹) and D (cm²s⁻¹) are the bulk concentration and Pd(II) diffusion coefficient, respectively.

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Furthermore, the CV in Figure 1 depicts a crossover between the cathodic and anodic voltammetric scans, characteristic of nucleation processes; therefore, the potentiodynamic analysis indicates that Pd deposition on the GCE occurs by a diffusion-controlled nucleation mechanism. On the reverse sweep of this CV, the palladium deposition occurs until the equilibrium potential, E_eq = −0.13 V, is reached ((second cross-over) on the CV at null current). After E_eq, the formation of a notable stripping peak, corresponds to the oxidation of the Pd deposited on the electrode. From the E_eq value and the applied potential, E, it is possible to calculate the nucleation overpotential, (η = E−E_eq).

Figure 2 shows a family of potentiostatic current density transients recorded in the system GCE/5 mM PdCl₂ in the choline chloride and ethylene glycol DES at different applied potentials within the cathodic peak zone shown in Figure 1. As previously described by Aldana-González et al.,³³ it is possible to obtain solely the palladium electrochemical response in the DES by subtracting the experimental j-t plots recorded in DES, without Pd(II) ions (named j_DES), related with pseudo-capacitive processes occurring during the double layer formed by the interface GCE/DES, from the plots obtained at the same conditions, but in the presence of Pd(II) ions (termed j_Pd-DES), see for instance the inset in Figure 2.

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Figure 3a shows the resulting family of current density transients after subtracting the respective blanks. It has been recently shown that the residual water in the DES can be simultaneously reduced on the growing surfaces of metallic centers, for instance during the Ni,^33,41 Co³⁵ and Cr³⁴ electrochemical deposition on different substrates. As shown in these works, a useful criterion to assess the occurrence of concomitant reactions during the catalytic metal centers electrodepositon (like protons or water reduction)^42,43 is to compare the experimental current density transients, normalized by means of the coordinate of their respective current density maxima (t_m, j_m), with the theoretical plots (j/j_m)² vs. t/t_m for instantaneous and progressive nucleation mechanisms developed by Scharifker et al.^44,45 If the experimental plots lie within the bounds of the theoretical limiting expressions, it is reasonable to assess that no concurrent faradaic simultaneous reactions are occurring on the surfaces of the growing metallic centers. According with this criterion, presently it can be noted from Figure 3b that there is no evidence to suggest the simultaneous reduction of residual water in the DES on the surfaces of PdNPs; thus, the experimental current transient will be analyzed with the theoretical framework, see Eq. 2, proposed by Heerman and Tarallo⁴⁶ for the analysis of current density transients involving the electrochemical nucleation with diffusion-limited growth. The inset in Figure 3a depicts an example of the capacity of the Heerman and Tarallo model to describe the experimental current density transient recorded during the electrochemical synthesis of PdNPs from the DES, by means of nonlinear fitting of Eq. 8 into the experimental data. The best-fit parameters obtained are shown in Table I. From parameter P₁, see Eq. 6, it was possible to calculate the Pd(II) ions diffusion coefficient in this DES as (2.77 ± 0.19) × 10⁻⁷ cm²s⁻¹. It is important to mention that Katayama et al.,⁴⁷ reported a value of 1.2 × 10⁻⁷ cm² s⁻¹ for the diffusion coefficient of the ion [PdCl₄]²⁻ dissolved in hydrophobic ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide at 25°C using the Cottrell's equation while Lanzinger et al.,⁴⁸ reported a value of 0.19 × 10⁻⁷ cm² s⁻¹ in the ChCl:EG eutectic mixture at 50°C using the Randles–Sevcik equation. Moreover, from Table I it can be noted that parameter P₂, which is directly proportional to the number density of active sites, N₀, increases exponentially with the applied potential, E, meanwhile the nucleation frequency, A, varies linearly with E.

where

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In these equations F is the Faraday constant, z is the number of electrons transferred, λ is the integration variable, M and ρ are the Pd atomic mass and density, respectively, F is the Faraday constant, A is the palladium deposit nucleation frequency and N₀ is the number density of active sites for palladium nucleation on the electrode surface. The numerical values a = 0.051314213, b = 0.47910725, c = 1.2068142 and d = 1.185724 correspond to the parameters of the polinomial approximation to the Dawson integral.

Therefore, Eqn. 2 can be parameterized as follows:

Once determined the values of the nucleation frequency A, the analysis of the ln(A) vs. η dependence as described on³⁷ allows determination of the fundamental kinetic parameters α (the transfer coefficient) and j₀ (the exchange current density), which in turn permit estimation of the critical nucleus n_c size and the Gibbs free energy of nucleation ΔG(n_c). The fitting of the ln(A) vs. η values in Table I to Eq. 9 is shown in Figure 4 and results in α = 0.013 and j₀ = 0.0012 A cm⁻². With these values, the overpotential-dependent values of the thermodynamic quantities ΔG(n_c) and n_c for the nucleation of Pd from the choline chloride/ethylene glycol DES can be estimated, as summarized on Table II.

With

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Where ν_a is the atomic volume, e is the elemental charge, z₊ is the charge number of the depositing ion, σ is the surface tension, k is the Boltzmann constant and T is the absolute temperature.

As observed before in ref,³⁷ the α values obtained for the deposition of Pd from DES are rather low (0.013) as compared with the values obtained for metal electrodeposition in aqueous media. However, as discussed in³⁷ and references therein, the electrodeposition of metals is quite a different phenomenon from the outer shell electron transfer reaction from which the transfer coefficient α is defined by the Marcus theory. In a media such as DES is reasonable to expect a high complexation of the metallic species, which in turn could determine that the intermediate transition state of the electron transfer reaction is more similar to the metal ion species in solution (or in adsorbed state on the electrode surface) thus, rendering an asymmetric energy barrier for electron transfer. On the other hand, the j₀ values obtained indicate some kinetic facility, which correlates with the information provided by the thermodynamic parameters of nucleation ΔG(n_c) and n_c, i.e., low values for the energy barrier for nucleation and the fact that surface active sites occupied with a reduced single metallic atom are already stable entities for further metal ion aggregation and nucleus growth (n_c values < 1).

SEM and XPS analysis

Figure 5 shows SEM images obtained on the GCE surfaces after applying a potentiostatic current transient at η = −0.35 V during 120 s. It is worthy of mention that this overpotential value was chosen because during the PdNPs early stages, the transient follows quite close the instantaneous nucleation mechanism (see Figure 3b), i.e., a fast saturation of available nucleation active sites, a fact consistent with the ΔG(n_c) and n_c values obtained and also with the empirical information depicted in the SEM images shown in Figure 5. In such conditions a high density of monodisperse small sized nuclei should be expected as effectively observed. Furthermore, from integration of this current density transient, the electric charge, q = 2.6 × 10⁻³ C, involved during the PdNPs formation was calculated and from this q value and the Faraday's first law of electrolysis, the mass of the deposited palladium was obtained (1.43 × 10⁻³ mg). Now, regarding these images, it is possible to note that PdNPs formation took place all over the substrate surface, displaying diameters of (41 ± 5) nm, then since the standard deviation is smaller than 15% it can be considered that the PdNPs are monodisperse in size. As can be noted from the size distribution histogram in Figure 4, the PdNPs follow closely a normal distribution. These results support the prediction regarding the instantaneous nature of the Pd deposition at the applied overpotential.

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As can be concluded from the XPS analysis, see Figure 6, taken on the surfaces shown in Figure 5, the PdNPs are mainly compounded by metallic Palladium, Pd(0), and a small amount of either, PdO or PdO₂. It is important to stress that the XPS spectra in Figure 6 are quite similar to those reported by Shrestha et al.,²⁴ for palladium electrodeposited from a 1-butyl-1-methylpyrrolidinium dicyanamide ionic liquid.

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Figure 7a depicts a CV recorded in the system GCE/PdNPs/0.1 M HClO₄ where the PdNPs were deposited on the GCE as indicated in SEM and XPS analysis section. The typical response due to palladium oxidation to form palladium oxides within the forward potential scan (between 0.55 and 1 V), its corresponding reduction during the backward scan (between 0.6 and 0.4 V) and the associated hydrogen processes region on metallic palladium (between 0.05 and −0.2 V) are clearly present in the CVs. This experimental evidence further supports the presence of PdNPs on the GCE after applying the potential step. Figure 7b shows a CV obtained in this same system but after bubbling CO during 15 minutes, followed by N₂ during 20 minutes at a constant potential of −0.2 V. During the first cycle, it is possible to note the formation of a sharp anodic peak (between 0.6 and 0.7 V) due to adsorbed CO striping off the PdNPs. The second cycle depicts the typical Pd response in acid media. From integration of the CO striping peak, the electroactive surface area of the PdNPs was estimated as (1.00 ± 0.05) cm², around 5 times bigger than the geometric area of the GCE.

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Electrochemical oxidation of formic acid

The catalytic performance of the PdNPs electrodeposited on the GCE surface, as indicated in SEM and XPS analysis section, normalized against the mass of electrodeposited Pd toward the formic acid electrochemical oxidation is presented in Figure 8a. The mass activity reached a maximum, 4200 mAmg_Pd⁻¹, at around 0.5 V. It is important to note that the poisoning tolerance, which is defined as the peak current ratio of the forward scan to the backward scan,⁴⁹ which is practically 1.0, strongly suggests that the PdNPs surfaces are not poisoned with formic acid oxidation products. Figure 8b shows chronoamperometries recorded at different applied potentials during formic acid oxidation, using the same electrodes as in Figure 8a. The mass activity transients are characterized by a decrease in the mass activity over time that has been attributed to the poisoning effects of the species formed during dissociative adsorption of formic acid.²⁶ From this figure, it is possible to note that the higher steady state mass activity corresponds to the plot recorded at 0.2 V (ca. 172 mAmg_Pd⁻¹). Table III depicts a comparison of the electrochemical performance of different PdNPs-based electrodes reported in the literature, toward formic acid oxidation. From this comparison it can be noted that the GCE modified with PdNPs, by means of potentiostatic electrodepositon from the choline chloride and ethylene glycol deep eutectic solvent at 298 K, performs an outstanding activity.

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Table III. Comparison of the mass activity (mAmg_Pd ⁻¹) of different electrodes toward the FAOR.

Electrode	NPs synthesis method	Potentiodynamic*	Potentiostatic*,**	Reference
GCE/NPs	Potentiostatic electrodeposition from DES	4200 ± 100	172 ± 2	This work
C/PdNPs	Moderate wet chemistry method	250	50	9
C/CuPdAuNPs	Moderate wet chemistry method	550	50	9
C/PdCoNPs	Simultaneous reduction reaction using NaBH4	250	50	12
C/PdNPs	reduction reaction using NaBH4	150	20	12
C/PdNi	Sonochemical reactions	2500	-	15
C/PdCo	Sonochemical reactions	2250	-	15
C/PdMn	Sonochemical reactions	2000	-	15
C/PdFe	Sonochemical reactions	2490	-	15
C/Pd	Sonochemical reactions	500	-	15
C/Pd	reduction reaction	500	100	16
MWCNTs/Pd	reduction reaction	600	90	16
Carbon cloth /Pd/PdCoNPs	Potentiostatic electrodeposition from aqueous solution	1580	400	26
Carbon cloth /PdNPs	Potentiostatic electrodeposition from aqueous solution	540	100	26

^*At the peak potential ^**at steady state condition